(1) Field of the Invention
The present invention relates to a heater unit which can be suitably used for purification of automotive exhaust gases, etc.
(2) Prior Art
In addition to hitherto known porous ceramic honeycomb structures, metallic honeycomb structures have recently drawn attention as a catalyst, a catalyst carrier or the like for conversion of the nitrogen oxides (NO.sub.x), carbon monoxide (CO) and hydrocarbons (HC) contained in the exhaust gases discharged from the internal combustion engines of automobiles, etc.
Meanwhile, as the regulation for exhaust gases has become stricter, it is strongly desired to develop a heater or the like which can reduce emissions during the cold start of engine.
As an example of the honeycomb structures, there is known, for example, a technique disclosed in Japanese Utility Model Application Kokai (Laid-Open) No. 67609/1988. The literature discloses a catalytic converter comprising (1) a main ceramic monolithic catalyst and (2) a metallic monolithic catalyst capable of generating heat when electrified, which comprises a metallic carrier and alumina coated thereon and which is provided upstream of the main monolithic catalyst but closely thereto.
In the catalytic converter disclosed in Japanese Utility Model Application Kokai (Laid-Open) No. 67609/1988, the metallic monolithic catalyst (2) is simply a preheater capable of generating heat when an electric current is passed from the center of the foil type metallic honeycomb structure to the outer periphery, and has no resistance adjusting means (that is, only the material, dimension and rib thickness of the honeycomb structure are specified and no resistance adjustment means is provided) and accordingly does not have an appropriate temperature elevation property.
Hence, the present applicant previously proposed, in Japanese Patent Application Kokai (Laid-Open) No. 295184/1991, a heater comprising a honeycomb structure and at least two electrodes for electrification of the honeycomb structure, attached to the honeycomb structure, wherein the honeycomb structure has a resistance adjusting means (e.g. slits) between the electrodes and the open ends of the slits (the portions of the slits at the periphery of the honeycomb structure) are filled with a zirconia type heat-resistant inorganic adhesive. The present applicant also proposed, in Japanese Patent Application Kokai (Laid-Open) No. 241715/1992, a method for holding a honeycomb heater in a casing by covering the periphery of the honeycomb heater with a metallic band via an insulating material (e.g. ceramic mat, ceramic cloth or the like). Further, Japanese Patent Application Kokai (Laid-Open) No. 241715/1992 discloses a method for forming an insulating protective film on the surface of said band or a ring by coating the band or ring with a ceramic (e.g. Al.sub.2 O.sub.3 or ZrO.sub.2) by flame spraying etc.
The present applicant further proposed, in Japanese Patent Application No. 167645/1991, a heater unit comprising (1) a resistance adjusting type heater wherein at least part of the resistance adjusting means is coated with an insulating ceramic (e.g. vitreous enamel) or a metallic spacer coated with an insulating ceramic is inserted into the open end of each slit (resistance adjusting means) and (2) a casing for holding the heater (1) therein, coated with an insulating ceramic.
In each of the above techniques, the resistance adjusting means is insulated and protected. Under severe driving conditions (vibration and thermal shock, in particular) of automobiles, however, the heater of Japanese Patent Application Kokai (Laid-Open) No. 295184/1991 may detach; and the heater of Japanese Patent Application kokai (Laid-Open) No. 241715/1991 may develop, owing to the vibration of horizontal and vertical directions, the deformation of heater, breakage of spacer and wearing of insulating mat. In the heater unit of Japanese Patent Application No. 167645/1991, the above problems are mostly solved; however, all the portions, through which an electric current is presumed to pass, must be coated with a ceramic and the technique is therefore not an easy one.
In order to make lighter the heater system including a battery and cables or achieve the higher purification of exhaust gas, it is desired to install a heater in the vicinity (e.g. manifold position) of engine so that such installment can heat an exhaust gas to a higher temperature and thereby can allow a catalyst to exhibit its catalytic activity more quickly. However, when a heater is provided in the manifold position, the vibration is severe (about 30 G); the main direction of the vibration is a vertical direction which is the same as the direction of the exhaust gas flowing through the heater unit; moreover, the thermal shock applied to the heater unit is larger.